This specification relates to assessing the cognition of a person, such as can be done based on results of a cognitive test that has been administered to the person.
Among the behavioral manifestations of Alzheimer's disease (AD) and other cognitive disorders are a decline in ability to perform tasks dependent upon learning, retention and retrieval or use of newly learned information acquired within the past two weeks. Item recall tasks have been widely used to assess this cognitive ability. Cognitive processes involved in item recall tasks include attention, encoding, storage into working memory in the prefrontal cortex, plus transfer, encoding and storage of items into short-term memory in the hippocampus and other medial temporal lobe structures. Items can be retrieved from working memory for periods of less than one minute, whereas they can be retrieved from short-term memory for periods between two minutes and two weeks. The items themselves can consist of words, sounds, numbers, characters, syllables, images, locations, or odors.
Item recall tasks are composed of a number of study trials and/or study-test trials, plus test trials, in which a subject learns and recalls a list of n items. A study trial is one in which a subject is presented a list of items in some specified order, and attempts to learn them. A test trial is one in which a subject is asked to correctly recall or identify the items. There are several types of these trials. A study-test trial is a study trial followed by a test trial. A delayed free recall test trial is one in which there is no study trial and the subject is asked, after some delay, to freely recall the list of items from previous study trials without being provided any cues, hints or reminders. A cued recall test trial is one in which the subject is given a cue, hint or reminder for each item from the study trials, and is asked to recall the item based on that information. A recognition test trial is one in which the subject is presented the list of items from the study trial intermixed with a list of items that were not presented in the study trials, and is asked to identify which items came from the study trials and which items did not.
Of those tasks, free recall tasks have become an important part of clinical testing for the detection of AD, which impairs free recall tasks in the earliest clinical stage of the disease process, also known as mild cognitive impairment (MCI). Clinicians and researchers have used free recall tasks for AD diagnostic purposes to detect early, monitor progression, assess severity of impairment, and measure treatment effect, with mixed results. In some cases, using aggregate performance scores, some researchers have failed to discover differences between study groups, such as finding no difference in AD or vascular dementia (VD) patients using the immediate free recall trials from the Consortium to Establish a Registry for Alzheimer's Disease (CERAD) test battery. In contrast, some other researchers have demonstrated that application of a Markov Model was able to identify different memory processes indicative of AD or VD from the CERAD immediate recall task data.